Paper is sheet material containing interconnected small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Paper typically is made from cellulose fibers, although occasionally synthetic fibers may be applied. Paper products made from untreated cellulose fibers lose their strength rapidly when they become wet, i.e., they have very low wet strength. Wet strength resin can be added to paper to produce stronger paper products. The types of wet strength resins that can be applied to paper may either be of “permanent” or “temporary” type, which are defined, in part, by how long the paper retains its wet strength after immersion in water.
Wet strength of paper is defined to be a measure of how well the fiber web holds together upon a force of rupture when in contact with water. Various techniques, such as refining of the pulp and wet pressing on the paper machine, can be used to reduce the strength loss of the paper upon wetting. The wet strength resins may improve the dry strength of the paper, as well. Wet strength improves the tensile properties of the paper both in wet and dry state by crosslinking the cellulose fibers with covalent bonds that do not break upon wetting. Wet strength is routinely expressed as the ratio of wet to dry tensile breaking force. Aldehyde functionalized polymers, such as glyoxylated polyacrylamide (GPAM), are widely used to increase wet strength.
During the papermaking process, aldehyde functionalized polymers, such as GPAM, are often added to the pulp suspension before paper sheet formation. Upon drying of the treated paper sheet the aldehyde functionalized polymer is believed to form covalent bonds with cellulose to increase paper dry strength and wet strength. Since the formation of covalent bond between the aldehyde functionalized polymer and cellulose is reversible in water, paper wet strength will decrease over time in water. As a result, the aldehyde functionalized polymers are also used as a temporary wet strength agent for tissue papers.
The strength performance of aldehyde functionalized polymers, such as GPAM, is known to be adversely affected by relatively high pH and high levels of alkalinity. In the absence of alkalinity, the aldehyde functionalized polymers are highly effective at acidic and neutral conditions. However, increasing pH of the aqueous solution to a value above 7 will result in significant strength loss. With alkalinity level of 50 ppm (CaCO3) or higher, the strength performance of aldehyde functionalized polymers, such as GPAM, is impaired even at neutral pH conditions.
The negative effect of pH and alkalinity limits the application of the aldehyde functionalized polymer in many paper grades.
Precipitated calcium carbonate (PCC) filler is often added to printing/writing paper for various benefits, such as for decreasing the cost and increasing opacity. The disadvantage is that carbonate ions from PCC dissolve in water, leading to high alkalinity and high pH of the pulp.
In addition, the application of the aldehyde functionalized polymer is also disadvantageous in many paper products produced using recycled pulps. This is because recycled paper often contains PCC and ground calcium carbonate (GCC). GCC originates typically from paper coating materials. Both PCC and GCC are re-introduced into the papermaking process and they both increase alkalinity of the system.
Papermakers often add strong acids to the pulp slurry during the papermaking process to enhance the performance of the aldehyde functionalized polymer. However, large quantity of acid is needed to lower the pH under high alkalinity conditions. Furthermore, lowering the pH of the papermaking water causes other issues, such as corrosion and compromise of process chemicals. Adding acid directly into pulp slurry results often in immediate precipitation or deposition of certain dissolved and suspended chemicals and particles. The handling of corrosive strong acids is also a safety concern for paper machine operators.
Therefore, there is a need to solve the problem for using aldehyde functionalized polymer effectively alone or together with other strength chemicals during papermaking, especially in those cases where the pH and/or the alkalinity of the pulp slurry is high.
Moreover, the aldehyde functionalized polymer is often applied on tissue paper grades to provide temporary wet strength. Upon drying of the treated paper sheet, aldehyde functionalized polymer is believed to form acetal covalent bonds with paper cellulose to increase paper initial wet strength. As the acetal bond formation is reversible in water it will decay over time. Consequently, aldehyde functionalized polymer products are often chosen over commercial permanent wet strength resins to improve paper repulping efficiency and also flushability in the sewage system.
As already discussed, GPAM performance is highly dependent on the wet end pH and alkalinity. Lowering pH and alkalinity facilitates acetal bond formation, leading to increased initial wet strength. Consequently, papermakers lower wet end pH to increase GPAM efficiency. The existing GPAM application methods may result in significant residual wet strength even when paper is in contact with water for an extended period of time i.e. permanent wet strength is obtained, especially under acidic wet end pH conditions. It would therefore be highly desirable to increase the wet tensile decay rate, as well, while still maintaining high initial wet strength performance.